Printers and copiers are well known. Modern copiers that utilize powder or liquid toners comprising toner particles to form visible images generally form a latent electrostatic image on an image forming surface (such as a photoreceptor), develop the image utilizing a toner (such as the aforementioned powder or liquid toners) to form a developed image and transfer the developed image to a final substrate. The transfer may be direct, i.e., the image is transferred directly to the final substrate from the image forming surface, or indirect, i.e., the image is transferred to the final substrate via one or more intermediate transfer members.
In general, the image on the final substrate must be fused and fixed to the substrate. This step is achieved in most copiers and printers by heating the toner image on the substrate. In some copiers and printers the fusing and fixing of the image is performed simultaneously with the transfer of the image to the substrate. This is achieved by utilizing a heated intermediate transfer member to perform the transfer and by pressing the intermediate transfer member against the final substrate. This combination of heat and pressure softens the toner particles and fixes them to the substrate.
These processes and fixers, intermediate transfer members other components and liquid toners suitable for carrying them out and printers utilizing these structures and processes are described in detail in U.S. Pat. Nos. 4,945,387; 5,047,808; 5,028,964; 5,089,856; 5,157,238; 5,286,948; 5,335,054; 5,497,222; 5,554,476; and 5,636,349; and PCT patent publications WO 96/17277, WO 97/07433, WO 99/61957 and WO 99/61958, the disclosures of all of which are incorporated herein by reference.
Particular reference is made to U.S. Pat. Nos. 5,047,808; 5,554,476 and 5,636,349 which describe a number of attributes of preferred intermediate transfer members suitable for liquid toner imaging.
U.S. Pat. No. 5,047,808 describes an intermediate transfer member comprised of a rigid core and an overlying intermediate transfer blanket. As described in the patent, a preferred intermediate transfer member provides a first transfer of images from an image bearing surface to the intermediate transfer member and a second transfer of the images from the intermediate transfer member to the final substrate. While both first and second transfers are performed under pressure, second transfer (which includes fixing and fusing of the image to the substrate) is performed under much higher pressure than first transfer. The patent teaches that the deformation per unit pressure during first transfer should be much lower than during second transfer. In other words, the intermediate transfer member should be “harder” for second transfer.
U.S. Pat. No. 5,335,054 provides a particularly advantageous method of achieving this desired characteristic of the intermediate transfer member. This patent describes an intermediate transfer member having two types of layers which contribute to this effect. In particular, the preferred intermediate transfer member as described in this patent has a soft, thin conforming layer, preferably formed of a soft polymer, and a sponge layer underlying the soft conforming layer. These layers provide conformance of the intermediate transfer member with the surface of the image bearing surface at low pressure and relatively low deformation and the desired stiffness of the intermediate transfer member under higher pressure conditions. Advantageously, a plurality of sponge and/or conforming layers are used to provide greater control over the compressibility profile of the member at first and second transfer.
U.S. Pat. No. 5,636,349 describes another desirable characteristic of intermediate transfer members. As described in this patent, the intermediate transfer member should be heated to a temperature at which the image on it adheres to the substrate. While the member is still pressing against the substrate the member is cooled sufficiently such that the cohesion of the image increases to such an extent that the image cohesion forces are greater than those causing adhesion to the member. When these conditions are met, the image is transferred in its entirety from the intermediate transfer member to the final substrate without leaving any appreciable toner residue on the intermediate transfer member.
It can be appreciated that this combination of requirements (and other requirements which have not been mentioned above) places very tight limitations on intermediate transfer members. While intermediate transfer members as described in the prior art can meet these requirements, the transfer parameters must be tightly controlled and the operating window available for these processes is limited. In state of the art systems the required transfer temperatures are provided by heating the drum on which the blanket is mounted, such that the image transfer surface is heated to a required temperature of 90 to 110 degrees Celsius. Higher or lower temperatures are also useful, depending on the polymers used in the toner particles, the carrier liquid used and the speed of the printing process. Since the blanket needs a sponge layer to provide some of the compressibility requirements of the member, and since sponges generally have high thermal impedance, the back of the blanket is much hotter than its transfer surface, often as much as 60–70 degrees hotter.
Not only does the blanket generally have to meet the stringent operating requirements mentioned above, but must also do so under high temperature, often much higher than the temperatures required for the actual transfer process. Furthermore, it has been found that the sponge layer is susceptible to damage from paper misfeeds or jams. When a number of sheets are fed together or jams occur, the sponge is sometimes compressed past its recovery point.
Furthermore, it has been found that intermediate transfer members exhibit short term memory effects under certain conditions. These effects manifest themselves in slightly different transfer characteristics for areas which carried an image on a previous transfer from areas which did not (background areas). It is believed that the memory effect is caused by variations in surface temperature on the transfer surface and/or by uneven absorption of carrier liquid from the liquid toner by a surface transfer layer of the transfer member. PCT patent publication WO 96/13760 and U.S. Pat. No. 5,592,269 provide at least partial solutions to these problems, at the cost of some additional system and/or toner complexity.
Reference is also made to U.S. Pat. No. 5,286,948, which describes a fusing apparatus and method utilizing a thin membrane as a fusing element. The membrane is mounted on two end elements to form a cylindrical drum of which the membrane forms the cylindrical surface. This element, which is generally too thin to support itself, especially during transfer, is supported by gas pressure within the drum and/or by mechanically applied pressure on the end elements to tension the membrane. It should be noted that the gas pressure itself also provides pressure on the end elements to tension the membrane.
PCT publication WO 00/31593, the disclosure of which is incorporated by reference, describes a roller, suitable for use as either a fuser or intermediate transfer member, in which a small amount of water or other liquid is placed in the interior of a roller formed by a thin membrane and two end plates. When the liquid is heated, the pressure in the roller caused by the evaporated liquid provides for a fuser suitable for fusing an image to a substrate and/or for an intermediate transfer drum.
EP 0 772 100 A2 describes a fuser roller in which vapor, evaporated from a heated liquid within a sealed roller, is used to heat the outer surface of the roller and fuse an image on a sheet against which the roller is pressed. The cylindrical surface of the roller is apparently rigid, since air is evacuated from the interior and the cylinder must be strong enough so that it doesn't collapse. Furthermore, the use of water as the liquid is not considered desirable, since the vapor pressure of water at the desired fusing temperature (190 degrees Celsius) is considered to be too high.
JP Publication 08320625 describes a fixing roller system in which the interior of a hollow roller is completely filled with water or oil. The liquid is heated and the roller is used as a fixing roller.
U.S. Pat. No. 4,172,976 describes heat rollers in which a relatively large amount of liquid having a relatively high vapor pressure, such as water or alcohol is contained. The liquid is heated via an intermediate conduction member situated between a heater at the center of the cylinder and the liquid.
The disclosures of the above referenced applications, patents and publications are incorporated herein by reference.